3GPP (3rd generation partnership project) has started studying to evolve existing networks to LTE (long term evolution)-based public safety solutions, which name is public safety LTE (PS-LTE). Public safety network provides communications for public safety services such as police, fire, ambulance, war, natural disaster, and so on. In general, public safety network is mainly used for mission critical text/voice/video communications among large number of users.
Mission critical push to talk (MCPTT) service in PS-LTE is intended to support one-to-one and one-to-many voice/video communications, where each user has the ability to gain an access to the permission to talk in an arbitrated manner. That is, MCPTT is a walkie-talkie-like service and the user can gain the permission to talk by pressing the ‘talk button’ to start talking Push-to-talk sessions are basically half-duplex communications: while one user speaks, the others should listen. Some floor control functionalities are required for controlling the talk permission.
There are two types of MCPTT network, i.e., On-network and Off network. The first one is On-network, which means that the radios talk through the existing network just as our cellular phones do. Meanwhile, MCPTT also considers the type of Off-network, which means that the radios talk directly with no help of network infra. Off-network MCPTT in Release 13 is defined as the collection of functions and capabilities required to provide direct communication between user equipments (UEs) (i.e., MCPTT clients) using proximity-based service (ProSe) discovery and ProSe communication path using evolved universal terrestrial radio access (E-UTRA).